The implicit duration of an anonymous span child of a sequential container is explicitly defined to be zero (section 10.4); however, the implicit duration of an explicit singleton (i.e., non-nested) span in the same context

Here is an example, where I ignore whitespace to aid in viewing. Given a fragment:

Here, anon2 is assigned an indefinite implicit duration because its parent is an explicit span which defaults to 'par' time containment semantics. Since SMIL requires a seq container that contains an indefinite duration child to also be indefinite, then that indefinite propagates up the node tree.

This seems counterintuitive. I propose to resolve by a slight change in 10.4 from the current text:

"if the anonymous span's parent time container is a parallel time container, then the implicit duration is equivalent to the indefinite duration value as defined by [SMIL 2.1]; if the anonymous span's parent time container is a sequential time container, then the implicit duration is equivalent to zero"

to read:

"if the anonymous span's nearest explicit time container ancestor is a sequential time container, then the implicit duration is equivalent to zero; otherwise, it is equivalent to the indefinite duration value as defined by [SMIL 2.1]"

and add the following Note

"Note: An element is an explicit time container if it specifies a timeContainer attribute."